1. Field of the Invention
The present invention relates to a cable of a defect detection sensor for inspecting heat tubes and a method of analyzing an insertion force of the cable, and more particularly, to an inspection cable enabling inspection of a defect in a slender heat tube by reducing frictional resistance generated when inserting a defect detection sensor into a tube of a helical heat exchanger, and a method of analyzing an insertion force required to insert the tube defect detection sensor into the tube of the helical heat exchanger.
2. Description of the Related Art
In general, a nuclear power plant produces electricity by supplying steam generated by a steam generator to a turbine. The steam generator performs heat exchange between a high-temperature high-pressure primary coolant flowing in a heat tube of a heat exchanger and a secondary coolant flowing outside of the heat tube. As a result of the heat exchange, the secondary coolant turns to steam.
The primary coolant circulating inside the steam generator is high-temperature high-pressure water. The primary coolant is contaminated by radioactive substances while heated and flowing through the nuclear reactor. Therefore, if the heat tube is broken, the primary coolant may contaminate the secondary coolant, causing leakage of radioactivity.
Since the heat tube of the steam generator forms a barrier between the primary coolant which is radioactive and the secondary coolant which is non-radioactive, an integrity inspection of the heat tube is strictly regulated by rules and standards with the regular in-service inspection (ISI) and schedule.
Generally, during an inspection for a heat tube of a steam generator of a commercial nuclear power plant and an inspection for a heat tube of a general heat exchanger, an eddy current test (ECT) probe is inserted in the heat tube to inspect whether the heat tube has a defect, accordingly determining soundness of the heat tube. For the inspection, an ECT sensor unit consisting of the ECT probe and a cable is necessary, which is appropriate for a size and a shape of the heat tube of the steam generator.
The cable is composed of signal cables and a protection cable. The ECT probe is connected at a leading end of the cable.
When the heat tube of the steam generator is a straight tube or has a few curved parts, friction resistance between the cable and the heat tube is low and, therefore, the ECT sensor unit is readily inserted into the heat tube.
An integrated nuclear reactor recently developed has the steam generator units inside its reactor vessel, and the steam generators are equipped with once-through type helical heat tubes.
Generally, a force to insert a defect detection sensor for inspecting the heat tube of the commercial nuclear power plant is proportional to the inserted length of the cable with sensor.
However, since the heat tube of the integrated nuclear reactor is a helical shape, the insertion force of the defect detection sensor exponentially increases, thereby causing a difficulty in inserting the defect detection sensor with a conventional inspection cable.
Accordingly, there is a desire for a method to analyze the insertion force of the cable in the helical heat tube so as to improve easiness of inserting the ECT probe into the helical heat tube.